In metal working, used lubricants and lubricant sludge accumulate which regularly have to be disposed of as waste. Used emulsions and water-containing lubricant sludge have to be separated into water, lubricant components and impurities at relatively high cost and conveyed for further disposal. In the particular case of lubricants and lubricant sludge, the waste contains valuable active substances. At the same time, in addition to the disposal costs, the prices for the charge materials, i.e. for mineral oils and additives, progressively increase. For this reason, methods for reprocessing oil-water mixtures for thermally recycling the oil concentrates and treating the weakly oil-containing water in biological clarification plants or recovering the water via reverse osmosis plants for producing emulsion or for other purposes become commercially interesting. An even greater commercial advantage is obtained if the purified oil concentrate can be returned to the lubricant cycle.
Evaporation, membrane filtration and the separator, decanter or tricanter techniques have, for example, been described for dewatering oil sludge. Eisenmann Maschinenbau GmbH & Co. KG thus provides a plant, in which firstly oil-containing residue is mechanically dewatered in a tricanter, then the oil phase is treated for use as fuel or for other purposes, the solid phase is burnt and the water phase is subjected to ultra-filtration.
Metal soaps, wear debris and other components, which contribute to a highly viscous residue or periodically to a highly viscous state during dewatering, lead to processing problems which are difficult to control. Very fine wear debris can barely, if at all, be separated from lubricating oil using gravity due to the small differences in density and particularly in the presence of surface-active substances (surfactants) and viscous medium.
In evaporator plants, the partly highly viscous consistency of the oils, which often reaches a maximum with dewatering, leads to adhesion and impeded evaporation of the water. With the methods described up to now, metal soaps which form in the emulsion cannot be removed from the oil. Attempts to remove the metal soaps with flocculation agents have been described in the literature.
Recovery of the additive-containing oil concentrate or lubricant concentrate by distillation, in which the unwanted metal soaps and tramp oil proportions remain in the residue, has not been described up to now and due to the high-boiling and partly thermally sensitive components was up to now considered impossible to perform.
High viscosity and impurities lead to deposits during batch distillation which hinder heat transfer and require a high level of maintenance.
With regard to dewatering the oil-water mixture and recovering the water by distillation in evaporator plants, it is to be noted, due to the relatively high evaporation energy of the water, that here energy recovery using compression condensation has proved advantageous. The low pressures of about 0.5 bar at which these evaporator plants are operated are not, however, sufficient for 100% dewatering which would be required for the oil to be subsequently reprocessed by distillation. These evaporator plants, moreover, achieve extensive dewatering by re-circulating the sump, which produces incrustation problems, poor heat transfer and a high level of maintenance, as well as a considerable thermal load on the oil components, and even destroys these in the process.
During a meeting of the Society of Tribologists and Lubrication Engineers (STLE) on Jun. 5, 2007, a method for recovering, purifying and reusing oil and water from roller emulsions using a vacuum evaporator at 100 to 300 mbar absolute and at a temperature between 50° and 70° was presented. A residual metal content of less than 400 mg/l is to remain in the product water. The oil obtained has a residual water content of less than 3%. A further reduction of the residual water in such a medium has up to now not been described. The separated water can potentially be reused to produce oil-in-water emulsions. Flocculation agents were proposed for separating wear debris and metal soaps from the oil concentrate. However, successful purification with flocculation agents strongly depends on the chemical composition and is comparatively cost-intensive with regard to chemical consumption and increased residual material formation.
Furthermore, dewatering of oil sludge using thin film evaporation at 145° C. and 500 mbar is known. However, this temperature and this low pressure are also not sufficient for a substantially complete dewatering to take place.
The recovery of lubricants and lubricant concentrates by distillation with high-boiling materials consisting of lubricant-containing sludge, for example oil sludge, is hampered by the partly highly viscous consistency, which is caused by wear debris, metal soaps, decomposition products, polymerised components and surface-active substances, such as polyglycols, and the high boiling state of materials and the thermal sensitivity, as well as incrustation and poor heat transfer associated with this.